Lubricating oil compositions for internal combustion engines typically include polymeric additives for improving the viscosity index of the lubricating composition, that is, modifying the relationship between temperature and the viscosity of the oil composition to reduce the temperature dependence of the viscosity, to lower the "pour point" of the composition, that is, to allow the composition to remain fluid at reduced temperature, and to provide "dispersant" properties, that is, to allow sludge particles to remain suspended in the oil composition.
Poly(alkyl methacrylate) (PMA) copolymeric additives and olefinic copolymer (OCP) additives are two classes of copolymers that are used as viscosity index improvers in lubricating oils. In general, PMA additives provide better low temperature performance than OCP additives, while OCP additives provide higher thickening efficiency than PMA additive, so that relatively less OCP additive is required to provide an equivalent thickening effect in the oil composition.
Dispersant properties may be imparted to PMA additives by incorporating monomeric units derived from nitrogenous comonomers into the copolymer, and may be imparted to OCP additives by grafting nitrogenous branches onto the OCP backbone. Some nitrogenous dispersant additives have been found to degrade fluoropolymer gaskets and seals. Since fluoropolymer gaskets and seals are enjoying increased acceptance in the automotive industry, there is a growing interest in non-nitrogenous dispersant additives.
PMA/OCP blends which provide a balance of the desirable properties of each type of additive are known. Coassigned U.S. Pat. No. 4,622,031 discloses concentrated blends of a nitrogen-containing PMA, an OCP and a "compatibilizer" graft copolymer having PMA branches grafted onto an OCP backbone, each dissolved in a hydrocarbon fluid. The compatibilizer copolymer stabilizes the thermodynamically incompatible PMA and OCP additives to discourage separation of the blend into discrete phases. U.S. Pat. No. 5,188,770 discloses a concentrated emulsion including a poly(alkyl methacrylate) copolymer and an olefin copolymer wherein alkyl methacrylate monomers are polymerized in an oil compatible liquid vehicle in the presence of an olefin polymer, hydrogenated isoprene, a hydrogenated butadiene-styrene copolymer, hydrogenated polyisoprene or hydrogenated polybutadiene.
While perhaps deceptively simple in theory, the development of a compatibilizer for stabilizing concentrated viscosity index improving blends of PMA and OCP copolymers is, in practice, a highly empirical undertaking.